Blender 2.82a very slow compared with 2.79
I had this same problem and I found a few ways to fix this. By default, the resolution % in 2.8 is set to 100%, whereas it was always 50% before. This only applies to renders however, not really anything else. You can find it in the Output tab underneath your dimension. Another thing you should do is make sure Blender is using your GPU. In your Render tab it may tell you it's using GPU compute but you need to go into your Preference settings and make sure it is using Cuda. I found all this out this morning and my render times went from 2 minutes to 12 seconds, and my playback is a lot smoother.